In the field of radio communications using high-frequency signals, required signals are separated from unnecessary signals by extracting signals of a particular frequency from a great amount of signals. The circuit that serves this function is generally referred to as filter and is mounted on many radio communications devices. A resonator of the filter that has a line structure is required to have a line length equal to about a quarter or a half of the wavelength at the resonance frequency. In addition, main design parameters of the resonator, such as the center frequency and the bandwidth, are fixed. As for the case where a radio communications device uses two frequency bands, the patent literature 1 by the present applicants discloses an exemplary device that has two resonators different in center frequency and bandwidth and a switch to switch between using one of the resonators and using the two resonators connected in series to each other.
In the variable resonator disclosed in the patent literature 1, as shown in FIG. 22, a first resonator 222 and a second resonator 223 are connected in series to each other via a switch 224 interposed therebetween on a surface of a dielectric substrate 220.
The first resonator 222 has a first line 225 having a length of L1 and second lines 226a, 226b, 227a, 227b, 228a, 228b, 229a and 229b having the same width W as the first line 225 and a length of Δh that are connected to the first line 225 and arranged at regular intervals ΔL on either side of the first line 225.
One end of the first line 225 extends for a length of L 3 to the direction away from the second lines 226a and 226b and is connected to a high-frequency signal input/output line 221 that extends in a direction perpendicular to the direction in which the first line 225 extends.
At the other end of the first line 225 opposite to the input/output line 221, a first line 270 of the second resonator 223 is disposed with the switch 224 interposed therebetween. The first line 270 has a length of L 2, and the end of the first line 270 opposite from the switch 224 is grounded. The first line 270 of the second resonator 223 also has second lines 230a, 230b to 233a, 233b arranged on either side thereof (four on each side) at regular intervals and connected thereto.
Line short-circuiting switches 250a, 250b to 255a, 255b are connected between free ends of adjacent second lines of the first resonator 222 and the second resonator 223. For example, the line short-circuiting switch 250a is connected between the free ends of the second lines 226a and 227a of the first resonator 222, and the line short-circuiting switch 250b is connected between the free ends of the second lines 226b and 227b. In other words, six line short-circuiting switches 250a, 250b to 252a, 252b are disposed symmetrically with respect to the first line 255 (three on each side of the first line 255).
Similarly, the second resonator 223 also has six line short-circuiting switches 253a, 253b to 255a, 255b connected between free ends of the second lines (three on each side of the first line). The line short-circuiting switches 250a, 250b to 255a, 255b are intended to change the effective line length (current path length, hereinafter referred to simply as path length) of the resonators using the property of the high-frequency current of flowing near the outer surface of a conductor (skin effect, described in detail later). If the line short-circuiting switch 250a connected between the second lines 226a and 227a is closed, the effective line length is reduced by 2Δh. Although not shown, a ground conductor is formed on the back surface of the dielectric substrate 220 at least over the regions opposing the input/output line 221, the first resonator 222 and the second resonator 223 to constitute a microstrip line structure.
A method of changing the resonance frequency of the first resonator 222 will be described. To minimize the resonance frequency of the first resonator 222, all the line short-circuiting switches 250a, 250b to 252a, 252b are opened (turned off). To slightly raise the resonance frequency from this minimum resonance frequency, one of the pairs of line short-circuiting switches 250a, 250b to 252a, 252b is closed (turned on). Then, compared with the line length in the case where all the line short-circuiting switches 250a, 250b to 252a, 252b are opened, the line length is reduced by 2Δh, and the resonance frequency is increased accordingly.
On the other hand, to further reduce the resonance frequency of the variable resonator from the minimum resonance frequency of the first resonator 222, the switch 224 is closed to connect the second resonator 223 to the first resonator 222 in series. With this arrangement, compared with the case where the first resonator 222 is used alone, the line length is elongated, so that the resonance frequency is reduced.
Patent literature 1: Japanese Patent Application Laid-Open No. 2005-253059 (FIG. 7)